1. Field of the Invention
The present invention relates to controlled airflow and air distribution within a laboratory safety enclosure and in particular, to turbulence-free airflow within a laboratory fume hood.
2. Description of the Prior Art
Fume hoods and laboratory safety enclosures are safety devices used in research, analytical, teaching, and other laboratories. These containment devices provide enclosed work areas where handling of toxic substances can be performed with minimum risk to users. They are used primarily in pharmaceutical, chemical, biological and toxicological laboratory settings.
Specifically, a laboratory safety enclosure such as a fume hood also known as a ventilated workstation is comprised of an enclosure or chamber within which materials are manipulated or worked upon by an operator, and an air exhaust mechanism for removing air from the enclosure.
The enclosure is comprised of a work chamber with an access opening and an exhaust or discharge opening. The enclosure may include a pair of spaced, parallel side walls; rear and upper walls joining the side walls; and a bottom wall or floor that together define the work chamber. The front edges of the side, upper and bottom walls define an access opening or inlet into the chamber through which the operator manipulates material within the chamber. Air also enters the chamber through this access opening as well as through a top or bottom bypass. The hood may also include a moveable closure sash to vary the size of the access opening. The air exhaust opening is preferably located on the opposite side of the chamber from the access opening, so that air flows across the chamber from the access opening to the discharge opening.
Analytically, a laboratory safety enclosure or fume hood is an exhausted enclosure, operating at a negative pressure relative to a room, which vents air away from a user and the laboratory. Generally, fume hoods are designed to maintain a high level of protection, provide a steady balance reading and to ensure that materials inside the enclosure are undisturbed by airflow.
Typically, air enters a fume hood's working chamber through one of three locations, either a sash opening, a top bypass, or a bottom bypass. A constant-speed fan and an automatically controlled variable damper regulate the volumetric flow rate of exhaust air, maintaining a constant face velocity for air entering the access opening of the work chamber. Back baffles are positioned such that air is exhausted directly from the fume hood's work surface as well as the top and center of the working fume hood chamber. Airflow pattern inside of the enclosure work area is controlled mainly by its geometry, sash opening height, face velocity at the inlet opening, operator presence in front of the sash opening, room air currents and very importantly by the geometry of any lab equipment placed inside of the work area itself.
Strict requirements are usually placed on fume hood operating configuration. These primarily include specification of face velocity and sash height ranges. It is generally believed that lowering sash height and increasing (within reasonable limits) face velocity would promote fume hood containment performance. At the same time, increasing face velocity above a certain level would actually compromise containment due to increased turbulence levels inside of the fume hood work area. It would also raise operating costs because of additional air supply demands. Proper fume hood operation therefore requires careful consideration of a variety of mutually dependent parameters.
Experimental smoke test observations as well as computer-predicted numerical simulations show a large vortex behind (downstream of) the bottom of the sash. Results also show the vortex to smoothly follow the back baffles almost to the top baffle in the working chamber. While vortex existence, consistently shown by both experimental and computer-simulated results is generally known, its effect on fume hood containment efficiency has not been addressed until the present invention.
The presence of this vortex results in a large-scale reversed-flow region in the immediate vicinity of the user work-area preventing efficient operation of a fume hood. Even worse, assuming a toxic compound is being handled inside the work area of a fume hood, a large zone with high concentration of toxic fumes is formed directly behind the front face of the hood. In fact, the leading edge of the reversed-flow region is located immediately behind the lower edge of the sash door, providing for a highly unstable containment performance.
Generally, fume hood operation demands a user to continuously perform various tasks inside the work area of a fume hood. These include weighing and measuring chemical compounds, calibrating experimental equipment and simply monitoring equipment performance. Frequent in-and-out hand movement is required to achieve these tasks. The highly unstable airflow balance directly behind the sash door opening is disturbed by this movement, causing highly toxic vapors contained in the reverse flow region to escape fume hood work area.
Moreover, if the sash door were moved to a higher position to facilitate fume hood work area access, there would be an immediate loss of containment due to the presence of the recirculation region directly behind the sash door. It is important to note that some of the highly toxic compounds are not only colorless, but also odorless as well.
Furthermore, increasing face velocity cannot eliminate the presence of the reverse flow region. Increasing face velocity would actually accelerate the roll, making the environment less stable. Increasing the sash opening height would simply make the roll smaller, unless the sash door is fully opened, in which case containment would be lost. Adding a top bypass slot would redistribute the roll, but as a practical matter it would make things worse by providing another potential escape avenue. Worse still, the bypass slot would be directly in front of the operator's face.
Invariably, fume hood design goals are achieved by minimizing turbulence intensity (level of flow fluctuations) characteristic of the airflow inside of a particular laboratory safety enclosure work area. Ideally, a turbulence-free design would provide for a smooth transition of airflow into the enclosure, moving air horizontally across the work surface. The resulting laminar flow structure would promote containment efficiency without affecting balance readings, dispersing light powders or otherwise compromising process efficiency. While turbulence intensity has been reduced by prior art design efforts, it has not been eliminated. What is needed is a fume hood design that allows for turbulence-free operation.